The equations of zero-sequence coupling transmission lines are derived from telegraph equations. Theory of in-terference method is introduced briefly

This method is a new one which can be used in measuring zero-sequence parameters of transmission lines with mutual inductance. The principle and method are used that the zero sequence parameters of overhead transmission lines are determined by using voltage and current vectors at the initial terminal

under the condi-tions of open and short circuit at the end terminal of overhead transmission lines. Interference method is first used to measure power frequency parameter of two 500 kV transmission lines in Northeast network. Measuring experiment performed on practi-cal transmission lines shows that this method is effective and easy to do on-site. The results meet the requirements of the transmission project.